There are two things about Benn is that scare me. 1) His lack of TDs in college...7 in three years! And 2) His 14.0 YPC average. Neither of these lead you to believe he is a big play receiver.
I watched this video on Thomas
http://www.youtube.com/watch?v=1nDhGrYAx5g
It starts out with an awesome block that leads to a TD. Throughout the video you see his physicality (stiff arms). He blocks well and doesn't go down with the first hit (doesn't run out of bounds). You also see him using great concentration and snagging the ball out of the air with his hands. His speed doesn't seem great (seems like a long strider) but he plays fast. He also seems to have a knack for anticipating where the defender will be after he catches the ball.
Of course these were mostly highlights and you can't see him getting off the line so I'm not sure how well he does that.
WRs are the hardest to predict. I've seen so many come out over the years that I was so sure would be great, only to see them out of the league in 2 or 3 years.